Hey thur, I know this may hurt a bit having your girlfriend call you bad names behind your back, however, you're both in love.
This may have been a joke between them, so first don't take it too far, she might not have meant it you know?
Confronting her about this, don't be too up front in case she didn't mean this she could be awkward or maybe offended. First I suggest asking her what she thinks about your moods and depression, find out what she says first then ask her "So you don't think I'm a ___? This will be the easiest way to go about this.
It is best that you ask her though because this will just eat at you if you don't and you might start to grow negative feelings towards the relationship. Reassure her that you aren't angry but you're confused and upset by what she said. Explain you weren't purposely listening to her conversation with her friend but you overheard and you don't regret it as what she said hurt you.
If she does think you're what she called you, then I suggest you don't stay together because that can be classed as emotional abuse and it will start making you more unhappy that you are, always put yourself first because if you don't, your depression will never start getting better.
I'm sorry to hear about this, but I'm here if you need advice or you need to talk, I know how hard this may be for you.
-Ally.